Recently, flat panel display devices such as a liquid crystal display device and an organic light emitting device have been actively developed due to their low energy consumption and light weight, and as their substrate material, glass has been generally used. However, glass is easily breakable and heavy, and difficult to produce as a thin film. In order to solve the problems, if a polymer film is used instead of glass, a flat panel display being light, thin, and flexible can be produced.
For example, US patent No. 2005/0203239A1 discloses that the glass substrate is replaced with composite compositions, that is, cured glass fiber/epoxy resin or cured glass fiber/acryl resin. Further, US patent No. 2005/0209404A1 discloses that the glass substrate is replaced with a polymer film using resins with a high glass transition temperature. However, in order to apply it to the general liquid crystal display device, a retardation film is needed in addition to the polymer film.
Generally, in a liquid crystal display, liquid crystal is injected between two substrates on which electrodes are formed, and the intensity of voltage that is applied to the electrodes is controlled to adjust the amount of transmitted light. The liquid crystal molecules have the anisotropy, and the anisotropy of liquid cells or films that include the molecules depends on how the liquid crystal molecules are distributed and an inclination angle of the molecules in respect to the substrate. Accordingly, in the case of the liquid crystal display, the amount and the color of light depends on the viewing angle.
It is required that the retardation film which has almost the same birefringence as the liquid crystal and the negative retardation is used to compensate the viewing angle.
In order to produce the retardation film that has the negative retardation in the thickness direction, a process of stretching a film, and a process of casting a substance having the birefringence are used. Currently, most retardation film is produced using the stretching process. However, since a stretching ratio is mechanically controlled, it is not easy to desirably control the angle, and the polarizing plates must be attached one by one while rolls are not used during the attachment by means of the polarizing plates. Accordingly, efficiency of the process is poor and it is difficult to control impurities.
Korean Patent Application No. 10-2005-0004439 discloses a process of casting liquid crystal having the birefringence. However, since the birefringence of the liquid crystal is very high and sensitive to the casting thickness, it is difficult to obtain a clear color display in the wide area.
On the other hand, in order to produce a retardation film having the thickness direction retardation, polyarylate has been used. If it is processed to a film type, the retardation becomes very high. Actually, its application to the liquid crystal display is impossible. Therefore, technologies capable of controlling the thickness direction retardation are needed.
For example, in U.S. Pat. No. 5,580,950, polyarylate is used. In the case of a film having a thickness of 50 μm, the thickness direction retardation becomes to be several hundred to several thousand nm. Therefore, it is hard to use as a retardation film of the liquid crystal display. However, an in-plane retardation can be easily controlled by the stretching process.
Further, Japanese patent application No. 2001-19749 discloses that the composition of monomers is changed to decrease the birefringence, however, which deteriorates high heat resistance that is an advantage of polyarylate.
Further, PCT/KR2005/003057 discloses that polyarylate has excellent light transmittance and heat resistance to increase gas barrier property, and it can be also applied as the polymer substrate material instead of glass to the flat panel display such as liquid crystal display. However, there is no report on a polymer film having a function of retardation film as a substitute of glass in the liquid crystal display.